Spectroscopy Instrumentation

Transcription

1 Chapter 1 Spectroscopy Instrumentation 1.1 Introduction Fourier-transform spectrometers (FTS or FT spectrometers) have been replacing the dispersive instruments in many infrared and near-infrared applications over the last couple of decades. Their inherent advantages compared with the dispersive instruments are proven and well accepted by scientists and engineers working in the field of spectroscopy. This chapter provides a general overview of the types of spectrometers commonly used today, focusing on the differences in their operating principles. An analysis of an FT spectrometer s advantages over its dispersive counterpart is then presented. 1.2 Types of Spectrometers Spectrometers can be categorized into three main types based on their principles of operation: dispersive, filter-based, and Fourier-transform instruments Dispersive spectrometers As the name suggests, dispersive spectrometers generate spectra by optically dispersing the incoming radiation into its frequency or spectral components, as illustrated in Fig Common dispersive elements include prisms and gratings. Dispersive spectrometers can be further classified into two types: monochromators and spectrographs. A monochromator uses a single detector, narrow slit(s) (usually two, one at the entrance and another at the exit port), and a rotating dispersive element allowing the user to observe a selected range of wavelength. Figure 1.1 shows the simplified schematic of a monochromator. A spectrograph, on the other hand, uses an array of detector elements and a stationary dispersive element. In this case, the slit shown in the figure is removed, and spectral elements over a wide range of wavelengths are obtained at the same time, therefore providing faster measurements with a more expensive detection system. 1

2 2 Chapter 1 1,2,3 Source and collimating optics Focusing optics and detector Figure 1.1 Schematics of a monochromator; a dispersive spectrometer. Narrow slits (an input and an output slit) are used to select a particular spectral element whose wavelength depends on the beam s incident angle on the grating. (Only the output slit is shown in this figure.) Filter-based spectrometers Filter-based spectrometers, or often simply called filter spectrometers, use one or more absorption or interference filters to transmit the selected range of wavelength, as illustrated in Fig As the beam passes through the filter, some of its spectral components are blocked through an absorption or interference process, while the desired spectral elements are transmitted. Various interference filters, from the ultraviolet through the far-infrared region, in various dimensions, are available as commercial-off-the-shelf items (e.g., Spectrogon AB, Taby, Sweden, and CVI Laser Corp., Albuquerque, NM, USA). A commonly used spectroscopic configuration is that of a filter-wheel system, also available commercially. This system consists of a number of filters (with different wavelength responses) placed near the circumference of a rotating wheel. A spectral band is selected by positioning the wheel so that the beam falls on a particular filter. With this configuration, however, only a few discrete bands can be selected, rather than a continuous spectrum as with a monochromator. Another variation of the filter-based systems is the tilting-filter instrument. 1 In this instrument a spectral band is selected by changing the incident angle of the beam on the filter. However, the wavelength tuning range is rather limited at about 3% of the center wavelength. Because of the limited number of discrete wavelengths in filter-wheel instruments and the limited range of wavelength in tilting-filter instruments, filter-based spectrometers are dedicated to the specific analyses for which they are designed.

3 Chapter 2 Signal-to-Noise Ratio To quantify the instrument s performance, the spectral signal-to-noise ratio (SNR) is used as the main measure throughout this book. The term has been used somewhat inconsistently; in some cases, it is used to quantify spectral repeatability, and in others, it is used to quantify spectral accuracy. Thus, it is appropriate to start with a clear definition of the term as it applies throughout this book. 2.1 Signal-to-Noise Ratio Defined In this book, the SNR measures the instrument s ability to reproduce the spectrum from the same sample, the same conditions, and the same instrumental configurations over a certain amount of time. This, in fact, is a measure of spectral repeatability, which measures the ability of the instrument to detect certain changes in the spectrum such as those caused by changes in the sample s spectral characteristics. Therefore, noise is the measure of the spectral deviations between measurements, regardless of the output spectrum s proximity to the true value. Accuracy, on the other hand, is a measure of the discrepancy between the actual measured value and the true standard or calibrated value. Wavelength accuracy of a spectrometer is crucial. It is important for the instrument to be able to conform to the calibrated standard in producing wavelength information within the instrument s intended resolution. For example, if the true absorption peak of molecule x is nm, the instrument should be able to give the correct peak wavelength information within its designed resolution. Thus, following the example, if the instrument has a designed resolution of 1 nm, then it should register the peak of molecule x at 4000 nm. This is required for proper information transfer between instruments. The spectral magnitude accuracy of FT spectrometers, however, is meaningless because it depends on various factors that are difficult to standardize, such as the optics transmission properties, the detector s characteristics, the speed of the moving mirror, the electronics bandwidth, among various other factors. However, this does not generally cause a problem 11

4 12 Chapter 2 because the instrument s transfer function is usually zeroed-out by first taking the measurement of a reference spectrum. The reference spectrum may simply be that of the ambient air, or a certain reference sample. Fourier-transform spectroscopic measurements generally involve two steps: first is the recording of the reference spectrum, and second is the recording of the sample spectrum. This should apply to both the absorption and the emission studies. Theoretically, the results are then independent of the instrument s transfer function, and are transferable between different instruments. For this to be true, however, the recorded intensity at each wavelength has to have high repeatability, and be within the linear range of the detection unit. The importance of the detector s linearity is discussed in Sec Quantifying Signal-to-Noise Ratio Most of the measurements using FT spectrometers rely on the single beam technique that involves taking the ratio of the sample s transmission spectrum to the reference spectrum at two separate times. Consequently, spectral noise N() can be computed according to the following equation: 1 T N a T, (2.1) b where T a and T b are the transmission spectra of the same sample/buffer taken at two different times, as illustrated in Fig This method can also be applied to the double-beam measurements, in which case T a and T b are taken simultaneously. In the case of zero noise, N() will be a flat line at zero across the wavelength range. A 100% line is defined by 100 T a () / T b () and is usually specified in the FT spectrometer s manufacturer s data sheet. The root-mean-square (rms) value of the spectral noise N rms can then be computed as N rms 1 n i N 2, (2.2) n i 1 where n is the number of spectral elements being observed. The range of the spectral elements should cover only the spectral range of interest. Note that the rms value is usually three to five times smaller than the peak-to-peak noise value. Finally, spectral SNR can be computed as 1 SNR. (2.3) N rms

5 Chapter 3 Principles of Interferometer Operation The interferometer is the core of every Fourier-transform spectrometer. Today s FT spectrometers use a variety of interferometer designs. However, they are all still based on the simple, yet historically most important, Michelson interferometer. In this chapter, the operating principle of the Michelson interferometer for FT spectroscopy is discussed. It is the objective of this chapter to provide a thorough physical understanding of how a spectrum is generated in an FT spectrometer. Enough mathematics is used to aid comprehension. First, a qualitative overview is provided, which is followed by a more detailed explanation starting with the wave description of light. Then, the factors that limit the output spectral resolution are explored, and finally, the interferogram processing techniques sometimes necessary to obtain accurate spectra are briefly discussed. 3.1 Overview Figure 3.1 shows a schematic of a Michelson s interferometer. It consists of a beamsplitter and two plane mirrors that are perpendicular to each other. One of the plane mirrors, M2, moves linearly in the direction shown by the arrow. As light enters the interferometer, it is amplitude divided at the beamsplitter. Approximately one-half of the light is transmitted and the other half is reflected. The transmitted and reflected beams are then reflected at mirrors M2 and M1, respectively. The beams are then recombined at the beamsplitter and detected by a photodetector. Under first consideration is light from a monochromatic source. When L 1 is equal to L 2, the two beams travel the same distance from the point they leave the beamsplitter to the point where they recombine at the beamsplitter. Being inphase, they interfere constructively (Fig. 3.2) and the detector sees a maximum intensity. As L 2 moves away from the zero optical path difference (OPD), the 17

6 18 Chapter 3 Fixed Mirror, M 1 Light source Beamsplitter Moving Mirror, M 2 Detector Figure 3.1 Schematic of a two-plane mirror Michelson interferometer. OPD Figure 3.2 Monochromatic waves at zero OPD. intensity starts to decrease as phase difference is introduced. Note that the OPD is equal to twice the mirror retardation distance because the OPD corresponds to the distance traveled by the beam to and from the moving mirror. When the OPD is equal to /2, destructive interference occurs as the two recombined beams become out-of-phase with each other. Thus, as the moving mirror travels at a constant velocity, the detector sees a sinusoidal-varying intensity. This sinusoidal signal is a function of mirror M2 displacement with a period of /2. The recombined interfering beams intensity fluctuation as a function of mirror displacement is called an interferogram. The interferogram is then Fourier transformed to obtain the spectrum. Figure 3.3 shows an interferogram from a monochromatic source and its spectrum.

TN-100 The Fundamentals of Infrared Spectroscopy The Principles of Infrared Spectroscopy Joe Van Gompel, PhD Spectroscopy is the study of the interaction of electromagnetic radiation with matter. The electromagnetic

Experiment #5: Qualitative Absorption Spectroscopy One of the most important areas in the field of analytical chemistry is that of spectroscopy. In general terms, spectroscopy deals with the interactions

Installation and User Guide 10Spec TM 10 Degree Specular Reflectance Accessory The information in this publication is provided for reference only. All information contained in this publication is believed

PURPOSE In this experiment we will use the diffraction grating and the spectrometer to measure wavelengths in the mercury spectrum. THEORY A diffraction grating is essentially a series of parallel equidistant

PUMPED Nd:YAG LASER Last Revision: August 21, 2007 QUESTION TO BE INVESTIGATED: How can an efficient atomic transition laser be constructed and characterized? INTRODUCTION: This lab exercise will allow

Raman Spectroscopy Basics Introduction Raman spectroscopy is a spectroscopic technique based on inelastic scattering of monochromatic light, usually from a laser source. Inelastic scattering means that

Introduction 1 Electromagnetic Spectrum The electromagnetic spectrum is the distribution of electromagnetic radiation according to energy, frequency, or wavelength. The electro-magnetic radiation can be

Interference Physics 102 Workshop #3 Name: Lab Partner(s): Instructor: Time of Workshop: General Instructions Workshop exercises are to be carried out in groups of three. One report per group is due by

Activity 17 Electromagnetic Radiation Why? Electromagnetic radiation, which also is called light, is an amazing phenomenon. It carries energy and has characteristics of both particles and waves. We can

How can I tell what the polarization axis is for a linear polarizer? The axis of a linear polarizer determines the plane of polarization that the polarizer passes. There are two ways of finding the axis

Measuring the Refractive Index of Infrared Materials by Dual-Wavelength Fabry-Perot Interferometry A Senior Project presented to the Faculty of the Physics Department California Polytechnic State University,

VISIBLE SPECTROSCOPY Visible spectroscopy is the study of the interaction of radiation from the visible part (λ = 380-720 nm) of the electromagnetic spectrum with a chemical species. Quantifying the interaction

Synthetic Sensing: Proximity / Distance Sensors MediaRobotics Lab, February 2010 Proximity detection is dependent on the object of interest. One size does not fit all For non-contact distance measurement,

Name: Periodic Wave Phenomena 1. The diagram shows radar waves being emitted from a stationary police car and reflected by a moving car back to the police car. The difference in apparent frequency between

SR2000 FREQUENCY MONITOR THE FFT SEARCH FUNCTION IN DETAILS FFT Search is a signal search using FFT (Fast Fourier Transform) technology. The FFT search function first appeared with the SR2000 Frequency

Ray Optics Minicourse COMSOL Tokyo Conference 2014 What is the Ray Optics Module? Add-on to COMSOL Multiphysics Can be combined with any other COMSOL Multiphysics Module Includes one physics interface,

Spherical Beam Volume Holograms Recorded in Reflection Geometry for Diffuse Source Spectroscopy Sundeep Jolly A Proposal Presented to the Academic Faculty in Partial Fulfillment of the Requirements for

Please do not remove this manual from from the lab. It is available at www.cm.ph.bham.ac.uk/y2lab Optics Introduction Optical fibres are widely used for transmitting data at high speeds. In this experiment,

UV/Vis Spectroscopy Varka Evi-Maria Ph.D. Chemist AUTH Thessaloniki 2012 Introduction of Spectroscopy The structure of new synthesised molecules or isolated compounds from natural sources in the lab must

Preview of Period 3: Electromagnetic Waves Radiant Energy II 3.1 Radiant Energy from the Sun How is light reflected and transmitted? What is polarized light? 3.2 Energy Transfer with Radiant Energy How

Northeastern University, PHYS5318 Spring 2014, 1 1. Introduction Experiment 5. Lasers and laser mode structure The laser is a very important optical tool that has found widespread use in science and industry,

Physics 41 Chapter 38 HW Key 1. Helium neon laser light (63..8 nm) is sent through a 0.300-mm-wide single slit. What is the width of the central imum on a screen 1.00 m from the slit? 7 6.38 10 sin θ.11

EXPERIMENT 11 UV/VIS Spectroscopy and Spectrophotometry: Spectrophotometric Analysis of Potassium Permanganate Solutions. Outcomes After completing this experiment, the student should be able to: 1. Prepare

Test IV Name 1) In a single slit diffraction experiment, the width of the slit is 3.1 10-5 m and the distance from the slit to the screen is 2.2 m. If the beam of light of wavelength 600 nm passes through

2008 AGI-Information Management Consultants May be used for personal purporses only or by libraries associated to dandelon.com network. Optical Metrology Third Edition Kjell J. Gasvik Spectra Vision AS,

Selecting Receiving Antennas for Radio Tracking Larry B Kuechle, Advanced Telemetry Systems, Inc. Isanti, Minnesota 55040 lkuechle@atstrack.com The receiving antenna is an integral part of any radio location

Diffraction and Young s Single Slit Experiment Developers AB Overby Objectives Preparation Background The objectives of this experiment are to observe Fraunhofer, or far-field, diffraction through a single

Using the Spectrophotometer Introduction In this exercise, you will learn the basic principals of spectrophotometry and and serial dilution and their practical application. You will need these skills to

Experiment (8) Using light scattering method to find The surface tension of water The aim of work: The goals of this experiment are to confirm the relationship between angular frequency and wave vector

Laboratory #3 Guide: Optical and Electrical Properties of Transparent Conductors -- September 23, 2014 Introduction Following our previous lab exercises, you now have the skills and understanding to control

EMC STANDARDS The EMC standards that a particular electronic product must meet depend on the product application (commercial or military) and the country in which the product is to be used. These EMC regulatory

Section 8 All materials, which are above 0 degrees Kelvin (-273 degrees C), emit infrared energy. The infrared energy emitted from the measured object is converted into an electrical signal by the imaging

Analysis of Riboflavin in a Vitamin Pill by Fluorescence Spectroscopy** Objectives In this lab, you will use fluorescence spectroscopy to determine the mass and percentage of riboflavin in a vitamin pill.

A down-under undergraduate optics and photonics laboratory Barry Perczuk and Michael Gal School of Physics, The University of New South Wales, Sydney, NSW 2052, Australia ABSTRACT Our senior undergraduate

Measuring of optical output and attenuation THEORY Measuring of optical output is the fundamental part of measuring in optoelectronics. The importance of an optical power meter can be compared to an ammeter

5.33 Lecture Notes: ntroduction to Spectroscopy What is spectroscopy? Studying the properties of matter through its interaction with different frequency components of the electromagnetic spectrum. Latin:

The Measurement of Sensitivity in Fluorescence Spectroscopy Among instrumental techniques, fluorescence spectroscopy is recognized as one of the more sensitive. In fluorescence, the intensity of the emission

Appendix I FV /26/5 SPECTROPHOTOMETRY Spectrophotometry is an analytical technique used to measure the amount of light of a particular wavelength absorbed by a sample in solution. This measurement is then

Introduction to Absorbance Spectroscopy A single beam spectrophotometer is comprised of a light source, a monochromator, a sample holder, and a detector. An ideal instrument has a light source that emits

Holography 1 HOLOGRAPHY Introduction and Background The aesthetic appeal and commercial usefulness of holography are both related to the ability of a hologram to store a three-dimensional image. Unlike

INSURANCE SCAM OPTICS - LABORATORY INVESTIGATION P R E A M B L E The original form of the problem is an Experimental Group Research Project, undertaken by students organised into small groups working as

II The Nature of Electromagnetic Radiation The Sun s energy has traveled across space as electromagnetic radiation, and that is the form in which it arrives on Earth. It is this radiation that determines

PROGRESSIVE WAVES 1 Candidates should be able to : Describe and distinguish between progressive longitudinal and transverse waves. With the exception of electromagnetic waves, which do not need a material

What s in the Mix? Liquid Color Spectroscopy Lab (Randy Landsberg & Bill Fisher) Introduction: There is more to a color than a name. Color can tell us lots of information. In this lab you will use a spectrophotometer